The manufacture of integrated circuits in a semiconductor device involves the formation of a sequence of layers that are categorized by their location in the front end of the line (FEOL) or in the back end of the line (BEOL). In BEOL processing, metal interconnects and vias form horizontal and vertical connections between layers and these metal lines are separated by insulating or dielectric materials to prevent crosstalk between the metal wiring. A popular method of forming an interconnect structure is a dual damascene process in which vias and trenches are filled with metal in the same step. Recent achievements in dual damascene processing include lowering the resistivity of the conductive metal by switching from aluminum to copper, decreasing the size of the vias and trenches with improved lithographic materials and processes, and reducing the dielectric constant of insulating materials to avoid capacitance coupling between conductive lines.
A lithography method in which a pattern on a mask is transferred into a photoresist on a substrate with an exposure step is typically used to define vias and trenches in the dual damascene structure. While most photoresists are optimized for FEOL applications that generally have smaller feature sizes or critical dimensions (CD) than BEOL layers, BEOL processes have unique challenges that require special solutions in some cases. For example, when forming a trench in a dual damascene structure, the lithography process usually must contend with considerable topography where the photoresist coating covers a planar surface and also fills a via hole. Furthermore, the imaging process can be negatively affected by solvents or amines in the dielectric materials that form the side of a via hole.
There is an ever present demand for smaller trench widths to be formed in order for devices with higher performance to be built. From a photoresist standpoint, a smaller CD is more easily printed when the film thickness is reduced or the exposure wavelength is decreased according to the equation R=kλ/NA. R is the minimum CD that can be resolved while k is a process constant, λ is the exposure wavelength, and NA is the numerical aperture of the exposure tool. Thinner films help to lower k on a planar substrate but when forming a trench over a via hole in a dual damascene process, a thin photoresist film is not effective because the resulting non-planar coating will cause problems in controlling the width of the trench above the via.
A bilayer concept has been introduced in an attempt to overcome the difficulties associated with imaging a thin single layer of photoresist over topography. Typically, the top layer in a bilayer scheme is a thin film of photoresist containing a small percentage of an element like silicon that can easily form an oxide in an oxygen plasma. The bottom layer is thicker so that it can form a planar surface over topography and often contains highly absorbing material that minimizes reflectivity to improve top layer patterning. In theory, the thin photosensitive layer on a planar underlayer should provide a path to forming a small trench over a via hole in a dual damascene structure. However, a lack of maturity in silicon containing bilayer resists has prevented widespread acceptance in the industry.
An option in the bilayer approach is to expose a silicon free photoresist layer over an underlayer and then selectively introduce a silicon reagent into either the exposed or unexposed regions. This method of treating a photosensitive film with a silicon compound that reacts to become incorporated into the film is called silylation. In U.S. Pat. No. 5,922,516, the underlayer is a photoresist that has been thermally crosslinked at a temperature between 110° C and 140° and a silicon compound in a vapor phase reacts with the top resist. However, selective incorporation of the silicon into either exposed or unexposed regions is difficult to achieve. A lack of maturity in silylation tools is another concern for this technique.
Another issue associated with photoresist processing is the process latitude of forming a pattern. It is important for a lithography process to have a large depth of focus (DOF) and a wide exposure latitude when implemented in manufacturing in order to reduce cost. DOF refers to the range of focus settings on the exposure tool that enable a feature to be printed within a specified tolerance, generally ±10% of a targeted linewidth or space width. Exposure latitude is the range of exposure doses that maintain the feature size within the ±10% specification. Variable reflectivity of the exposing radiation off the substrate must be controlled in order to achieve the feature size specification. One widely used technique is to form an anti-reflective coating on the substrate before coating the photoresist film. This bottom anti-reflective coating (BARC) is usually about 300 to 1000 Angstroms thick and is baked above 200° C. so that it does not interact with the photoresist by mixing with or outgassing into the overlying layer. These thin BARCs are employed when the substrate is relatively flat and have been tuned so that their refractive indices match a particular family of photoresists. For example, one type of BARC is available for 193 nm photoresists and another type has been developed to improve imaging of Deep UV photoresists. Once the photoresist pattern is formed, it must be transferred with an etch process through the ARC.
In order to achieve finer resolution in photoresist patterns, the exposing wavelength (λ) has been steadily shifting lower in recent technology generations or nodes. Above the quarter micron (250 nm) generation, i-line (365 nm) or g-line (436 nm) exposure tools are more popular because of a lower cost of ownership. For the 130 nm to 250 nm nodes, Deep UV (248 nm) exposure tools have been implemented as state of the art. Meanwhile, 193 nm exposure tools are thought to be the best solution for reaching the 100 nm node and a 157 nm technology is being developed for the 70 nm node.
With the shift to 248 nm and 193 nm wavelengths, a new lithography concept was introduced in which photoresists operate by a chemical amplification mechanism whereby one molecule of strong acid is capable of causing hundreds of chemical reactions in an exposed film. A strong acid is generated by exposing a photosensitive component and the acid reacts with acid labile groups on a polymer in positive tone photoresist. In negative tone photoresist, the acid initiates a crosslinking reaction. This enables a higher photosensitivity (faster photospeed) that increases throughput compared to the old mechanism where one photon caused one chemical event in the photoresist film. It should be noted that in positive tone photoresists, the exposed regions are washed away in an aqueous base developer while in negative tone photoresists the unexposed regions are washed away in the developer solution.
Unfortunately, this new chemically amplified approach is quite sensitive to traces of base compounds such as airborne amines or amines that diffuse into the chemically amplified photoresist from adjacent or underlying layers. Sometimes, amine concentrations as low as parts per billion (ppb) can inhibit or “poison” the chemically amplified reaction enough to prevent a pattern from being formed. At other times, the poisoning is less severe and appears as scum on the substrate where a thin film of photoresist is not washed away in exposed (positive tone) or unexposed (negative tone) regions. Even mild cases of scumming can be difficult to remove by further processing and as a result the substrate must be reworked by stripping the photoresist, recoating and re-exposing. Rework is expensive and a better alternative is to implement a photoresist process that does not form scum.
A prior art method of forming a dual damascene structure as described in U.S. Pat. No. 6,319,821 is shown in FIGS. 1a–1c. In FIG. 1a, an opening 18 has been formed in photoresist 17 on a stack consisting of passivation layer 16, dielectric layers 13 and 15, etch stop layers 12 and 14, and substrate 10. Etch stop layers 12 and 14 are comprised of a material like Si3N4 while passivation layer 16 which relieves stress in phosphosilicate dielectric layer 15 is a material such as SiOXNY. The opening 18 is etch transferred through underlying layers 13, 14, 15, and 16 to form a via hole 22. After photoresist 17 is stripped, a second photoresist 19 that is an i-line (365 nm) sensitive material is coated to fill via hole 22. Photoresist 19 is etched back to a level that is coplanar with etch stop 14. Then a Deep UV photoresist 23 is coated on passivation layer 16 and fills via hole 22 above photoresist 19. A trench opening 20 is formed by patterning photoresist 23 in FIG. 1b. Opening 20 is etch transferred through passivation layer 16 and dielectric layer 15. Photoresists 19 and 23 are then removed with a wet strip process and a metal layer 21 comprised of copper or aluminum is deposited to fill via 22 and trench 20 openings. After a planarization step, the completed damascene structure appears as illustrated in FIG. 1c. While this method reduces the effect of photoresist poisoning during trench formation by employing a chemical barrier that protects photoresist 23 from amines that might diffuse out of etch stop layers 12 and 14 and dielectric layer 13, it does not protect photoresist 23 from amines such as ammonia contained in dielectric layer 15 and passivation layer 16.
In another prior art example, U.S. Pat. No. 6,340,435 teaches how to perform selective etches of adjacent dielectric layers having different dielectric constants to form vertical and horizontal interconnects. However, the method does not mention how to overcome photoresist patterning concerns when forming trenches in dual damascene structures.
Another method of forming a dual damascene structure is found in U.S. Pat. No. 6,184,128. One key feature is that the trench pattern is formed on a dielectric layer that has not been patterned. In addition, an ultra thin photoresist of <1500 Angstroms is used for patterning. One drawback is that the photoresist can be coated on nitride layers that can be sources of amines which will contaminate chemically amplified resists. Since there is no BARC or chemical barrier layer between the photoresist and the nitride, there is a high likelihood of photoresist poisoning or scum formation.
Therefore, an improved patterning method for forming trenches in dual damascene structures is desirable in which the photoresist does not come in contact with any passivation layer, dielectric material or etch stop layer that might cause scum. Moreover, the method should not depend on immature silicon-containing photoresists in a bilayer scheme. It should avoid silylation techniques in which selective introduction of silicon into a film is difficult to control. The method should be easily implemented in manufacturing and be a low cost solution. Ideally, the method should be versatile by being applicable to all wavelengths of exposing radiation.